Improving the efficiency, consumption, dissipation and/or linearity of radio-frequency (RF) solid state power amplifiers is a high priority for manufacturers of ground mobile telephony network infrastructures due to the very demanding requirements for 3G-UMTS. The very high linearity required dramatically impacts the efficiency of power amplifiers used in base-stations. Similar requirements arise in space applications.
It is well-known; however, that linearity and energetic efficiency are somehow contradictory requirements.
The “flexible amplifier” concept consists in adjusting the power supply to the average RF power in order to achieve both high linearity and high efficiency. “Envelope tracking” is an evolution of this approach, wherein the power supply is adjusted in a dynamical way, following the RF signal envelope.
FIG. 1 illustrates the conceptual scheme of a RF power amplifier with envelope tracking. SRF(t) is an input RF signal (S-band, i.e. 2 to 4 GHz) with a bandwidth of e.g. 36 MHz which enters the amplifier. This signal is split into two components SRF1(t) and SRF2(t) by input coupler ICP. Envelope detector ED extracts the envelope E(t) from signal SRF1(t). The envelope signal E(t) drives a DC/DC converter DCC (linear and/or switching) which provides the high-power RF amplifier HPA with a time-varying power supply at voltage V0(t). Signal SRF2(t) enters the high-power RF amplifier HPA to be amplified. Delay line DL introduces an adjustable delay τ in order to ensure that SRF2(t−τ) and E(t) are synchronized so that, at any time, the power supply is adjusted to the amplitude of the RF signal to be amplified. The amplified output signal is indicated by reference Sout(t).
The main difficulty of the scheme of FIG. 1 resides in the implementation of the DC/DC converter. Linear converters are penalized by their low efficiency. On the other hand, the use of switching converters requires operating a power switch at a very high frequency—10 to 80 MHz for an envelope bandwidth of 5-10 MHz. Conventional Buck and Boost switching circuits, based on Si MOSFET, operate at much lower frequencies, typically around 100 kHz; operating them at a frequency of several MHz would introduce unacceptable switching losses.
For this reason it has been suggested to use a combination of a switching converter and a linear converter. The switching converter operates at a comparatively low frequency, and cannot follow the fast variations of the envelope, but it provides an average power supply with a high efficiency. The linear converter is less efficient but much faster, and assists the switching converter when the envelope changes at a fast rate. See e.g. Draxler, P. Lanfranco, S. Kimball, D. Hsia, C. Jeong, J. van de Sluis, J. Asbeck, P. M., “High Efficiency Envelope Tracking LDMOS Power Amplifier for W-CDMA”, Microwave Symposium Digest, 2006. IEEE MTT-S International, 2006.
European patent EP1214780 discloses a RF power amplifier wherein envelope tracking is achieved by switching from different individual power supplies at different DC levels. This implies additional complex circuitry and additional power lines, and therefore an increase of the weight and cost of the apparatus, which is detrimental especially for space applications.
U.S. Patent Application US 2008/111631 discloses a RF power amplifier wherein envelope tracking is provided by a simple, one-transistor, switching DC-DC converter providing a time-varying power supply to a one-transistor RF power amplifier. In order to achieve a sufficient envelope bandwidth, the transistor of the DC-DC converter is indeed a RF power transistor. In a particular embodiment, the transistor of the DC-DC converter and that of the RF power amplifier are identical, discrete transistors embedded in a common package. In this apparatus, the bottleneck is constituted by the diode, which is also an essential part of the DC-DC converter. Indeed, commercially available power rectifiers, able to deal with currents greater or equal to 1 A and powers greater or equal to 10 W, can only operate at switching frequencies of a few kHz, or hundreds of kHz. Conversely, RF rectifiers operating at several MHz, tens of MHz or even GHz can only deal with low currents (in the range of mA) and low power (less than a few W).
Moreover, the apparatus of US 2008/111631 raises concerns regarding the high level of parasitic reactive elements. Indeed, while the two transistors used as RF amplifier and switch of the DC/DC converter are efficiently and compactly embedded in a single package, the wires connecting said transistors with the diode, capacitor and inductor of the converter introduce high parasitic capacitances and inductances reducing the performances of the switching cell.